1. Field
The present embodiments relate to power sources for electronic devices. More specifically, the present embodiments relate to a parallel battery architecture with a shared bidirectional converter.
2. Related Art
Typical high-powered battery designs include battery packs that contain battery cells connected in various series and parallel combinations. For example, a six-cell battery pack of lithium cells may be configured in a three in series, two in parallel (3s2p) configuration. If a single cell can provide a maximum of three amps with a voltage ranging from 2.7 volts to 4.2 volts, then the battery pack may have a voltage range of 8.1 volts to 12.6 volts and provide 6 amps of current.
However, existing battery pack architectures have a number of drawbacks. First, a cell's state-of-charge or capacity may be difficult to calibrate. The best technique for determining state-of-charge is to measure the cell's open circuit voltage, which often correlates well, after temperature correction, with the state-of-charge for typical cell chemistries. However, open circuit voltage measurements may only be accurate when the pack is not being charged or discharged, which can be difficult for battery packs that discharge a small amount of current when at rest. In addition, calibration of the battery's capacity may require the user-initiated action of periodically resting the battery at different states of charge.
A second disadvantage relates to cell balancing. In any series configuration of cells, it is important that all cells be at the same state-of-charge (e.g., balanced). An imbalanced battery pack may have reduced capacity, because the cell with the highest state-of-charge terminates the charging cycle and cells with lower state-of-charge never get fully charged. When discharged, the cell with the least charge disables the pack, even if charge remains in other cells. Many cell-balancing techniques exist, but these techniques often rely on either accurate cell impedance measurements or accurate state-of-charge measurements. Battery packs identified as too far out of balance are typically identified and disabled, thus leading to reduced pack lifetime. A number of factors may further contribute to cell imbalance, such as differences between the aging characteristics of cells and/or uneven temperature distribution within the battery pack.
A third disadvantage of standard battery packs relates to the inefficient conversion of power from one DC voltage to another. Most voltages required in modern computing devices are much lower than the voltage provided by battery packs configured in series. Moreover, conversion of the battery pack voltage to the lower required voltages using buck converters and/or other down conversion techniques typically achieves 95% efficiency or worse. Note that buck converter efficiency may be related to the amount of decrease in voltage required, so battery packs with a series configuration and high voltages may waste significant power in converting to much lower voltages and higher currents.
Hence, what is needed is a battery pack topology that allows for accurate calibration of cells, improves battery lifetime, and avoids issues associated with cell balancing.